FIG. 1 shows a longitudinal section through a known configuration for a manually shifted gearbox 1 for a truck. The front parts of the gearbox are shown to the left in FIG. 1, which is to say, towards the vehicle engine. The rear parts of the gearbox are shown to the right in FIG. 1. The gearbox comprises (includes, but is not necessarily limited to) a basic gearbox 2, having a plurality of (multiple) possible gear ratios, and an auxiliary gearbox, commonly referred to as a range-change gearbox 3. The range-change gearbox 3 has a gear with a high gear ratio and a gear with no gear ratio; that is to say, a direct transmission configuration. The range-change gearbox 3 essentially doubles the number of gears provided by the basic gearbox.
The basic gearbox 2 shown in FIG. 1 comprises an input shaft 4, an intermediate shaft 5 and a main shaft 6. The latter is provided with a number of gears supported on bearings, so-called runners 7, 8, 9, 10 and 11. These can be rotationally locked to the main shaft by means of toothed clutch devices 12, 13 and 14. Corresponding gears on the intermediate shaft 5 are very often incorporated into the intermediate shaft 5 and are then referred to as racks. The toothed clutch devices can be synchronized or unsynchronized. In FIG. 1, the toothed clutch devices 12 and 13 are synchronized and the toothed clutch device 14 is unsynchronized. An unsynchronized toothed clutch device is generally significantly shorter in its radial extent than one that is synchronized. According to the example shown in FIG. 1, the transmission casing comprises a clutch cover 15, a basic transmission casing 16 and a range transmission casing 17.
Normally, at least one of the runners on the main shaft is used for reverse gears. With reverse gear engaged, the main shaft must have the opposite direction of rotation compared to when a forwards gear is engaged. As illustrated in FIG. 1, this is commonly achieved by one runner 11 not meshing with the opposite rack 18 on the intermediate shaft 5. Instead, an intermediate gear, a so-called reverse intermediate gear 19, is arranged above the plane shown in FIG. 1 (the reverse intermediate gear is therefore not visible in FIG. 1). FIG. 2b demonstrates this principle. In FIG. 2b, the gears 11, 18, 19 are shown as viewed from the left-hand side of FIG. 1 (i.e. from the front). FIG. 2a shows the corresponding principle for the forward gears (that is 7, 8, 9, 10 in FIG. 1). It must be noted that the runner 11 for reverse gears is coupled to the main shaft 6 by means of the unsynchronized toothed clutch 14 (see FIG. 1). Between the runners 10, 11 and the intermediate shaft 5, there is a relatively large amount of space around the intermediate shaft 5. This is because the toothed clutch 14, with its clutch fork 14a, requires space between the gears 10 and 11.
The same gearbox as in FIG. 1 is shown in a partially cut-away, three-dimensional perspective view in FIG. 3. In FIG. 3, it is possible to see the reverse intermediate gear 19, which is rotatably supported on a reverse gear shaft 20. The reverse gear shaft 20 is in turn fixed partly in the wall 21 of the basic transmission casing 16 and partly in an integrally cast projection also commonly referred to as a reverse gear shaft lug 22 (see FIGS. 1 and 3).
FIGS. 4a and 4b show an embodiment of the basic transmission casing 16 from a gearbox 1 such as illustrated in FIG. 1. FIG. 4a shows a side view of the casing 16, and FIG. 4b shows a view from the front which corresponds to being from the left in FIG. 1. Known cost-effective casting methods, such as pressure die casting, make it advantageous to design the basic transmission casing 16 with an opening 23 (see FIG. 4a) in one side wall thereof where the reverse intermediate gear 19 is arranged immediately inside the opening 23. The part of the reverse gear shaft lug 22 pointing towards the reverse intermediate gear 19 can thereby be formed by the casting of a die-cast part which is inserted into the opening 23. In an assembled gearbox 1, the opening 23 is covered by a cover.
The use of a so-called forced-feed lubricating oil system in order to increase the service life of gearboxes for heavy trucks and buses is already known. An example of a known lubricating oil system is shown in FIG. 3. This usually comprises an oil pump 24, which among other things, supplies bearings in the gearbox 1 with an oil flow for lubrication and cooling. The oil pump 24 may be located and driven in various ways. Three known and commonly encountered locations include: (1) The oil pump 24 can be arranged at the front end of the intermediate shaft 5. An example of an oil pump 24 located at the front end of an intermediate shaft 5 is shown in FIG. 5. Since the greater part of the oil flow must be delivered to the range transmission 3 (in the rear part of the gearbox 1), it is inconvenient to have the oil pump 24 arranged in the front part of the gearbox 1. Furthermore, this location can result in a shortage of space since the oil pump 24, a control 26 for the clutch and, in automatic gearboxes, a so-called gearbox brake must share the space in the front part of the gearbox. (2) The oil pump 24 can also be arranged at the rear end of the intermediate shaft 5; that is to say, on the right-hand side of the intermediate shaft 5 (the location not being shown in the Figures). In this configuration, the oil pump 24 is more centrally located at the rear end of the intermediate shaft; i.e., closer to the consumers of the oil flow. Unfortunately, however, such a location means that the range transmission 3 must out of necessity be shifted rearwards, with the result that the gearbox 1 as a whole becomes longer. (3) The oil pump 24 can be arranged at the rear end of the reverse gear shaft 20 (see the example of FIG. 3), driven by a separate shaft, and which is arranged coaxially inside, and passing through the reverse gear shaft 20. A gear 25, which meshes with one of the racks on the intermediate shaft 5 for driving the pump 24, is rotationally fixed to the separate shaft at the front end. The length of the gearbox need not be affected if the oil pump 24 is located at the rear end of the reverse gear shaft 20. The range transmission 3 can, however, find limited space in a radial direction. The oil pump 24 may also be in the way of any additional brake, for example, a retarder, which is often incorporated in the range transmission casing 17. Moreover the separate drive with separate shaft and separate gear 25 is relatively costly.
Furthermore, the oil pump 24 should draw oil as far away as possible from the walls (the front wall, rear wall 21 and the two side walls) in the basic transmission casing 16, and as far down as possible. This ensures that the oil pump 24 does not draw in air even if the vehicle is inclined in any direction. This also means that a separate line 27 (see FIG. 1), often referred to as a suction pipe, has to be used in connection with the aforementioned locations of the oil pump 24.
In order to protect the oil pump 24 from larger particles in the oil, the suction pipe 27 is often provided with a mesh, or so-called suction strainer, arranged at the inlet to the suction pipe 27. In order to increase the service life of the gearbox and to minimize the number of oil changes, the lubrication system is usually provided with an oil filter 28 (see, for example, FIG. 3). This is usually arranged so that the oil flow from the pump 24 passes through the filter 28 on the way to the lubrication points.
At low temperatures, the oil is viscous. A forced-feed oil flow can then give rise to very high oil pressures. In order to prevent damage to and loss of constituent parts and/or major leakage, the lubrication system is often provided with a pressure-limiting valve, which in FIGS. 6a, 6b and 6c is generally denoted by 29. The function of such a valve is shown in a simplified form in FIGS. 6a, 6b and 6c. 
A cone 30 is pressed against a seat 60 by means of a spring 31. FIG. 6a shows the valve 29 at normal pressure. The valve 29 is closed and all lubricating oil is led directly from the pump to the lubrication points. The arrow 61 symbolizes the oil flow from the oil pump towards the lubrication points. At a certain oil pressure the spring 31 is no longer capable of pressing the cone 30 against the seat 60. The valve 29 is opened and a part of the oil flow is then passed through the valve 29 (see FIG. 6b and flow arrows 63). The flow to the lubrication points is then less than normal (see flow arrow 62). The pressure in the bypass oil falls when it passes the gap between the cone 30 and the seat 60. This produces a temperature rise, which makes the lubricating oil more thin-bodied. It is then advantageous to return this part of the oil flow to the inlet side of the oil pump 24 (see FIG. 6c and flow arrow 64). The oil flow 65 coming from the pump 24 then becomes more thin-bodied. The oil pressure gradually falls and the pressure-limiting valve 29 is closed. This occurs more rapidly than if the bypass flow through the valve 29 were not returned to the inlet of the pump 24. Return directly to the oil pump therefore means that maximum oil flow 66 to the lubrication points is achieved after a shorter time.
There is thus a need for a better designed lubrication system in a gearbox of the aforementioned type, and in which the aforementioned disadvantages are mitigated or eliminated.